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The Global Nitrogen (N) Cycle and Carbon Sequestration Fact: Humans have more than doubled the rate at which reactive N is added to the biosphere annually. Fact: Through fertilizer production, the planting of legumes, and high temperature combustion, humans transform about 150 TgN annually from unreactive N to reactive N. Fact: A fraction of this reactive N enters ecosystems in which carbon sequestration is N limited. Question: Is N-stimulated carbon sequestration currently a significant component of the global carbon budget?

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Approach We used a process-based biogeochemistry model running with an open N cycle to estimate the annual N- stimulated carbon sequestration rate in North America. The current spatially explicit N- deposition pattern for the region was estimated using the results of Dentner and Crutzen.

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Carbon Sequestration in North America (PgC) N- Other All stimulated Mechanisms USA Canada Other North America

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Conclusions We estimate that N-stimulate carbon sequestration accounts for about ¼ of the current C sink in North America. Other important mechanisms include land cover and land use changes and CO 2 fertilization. Biogeochemistry models used in carbon-cycle studies must include coupled C-N cycles.